Area Forecast Discussion
Issued by NWS San Diego, CA
Issued by NWS San Diego, CA
831 FXUS66 KSGX 231641 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 941 AM PDT Sun Jun 23 2024 .SYNOPSIS... Hot conditions will continue today, with widespread moderate to locally major heat risk. Chances of showers and isolated thunderstorms this afternoon for the mountains and deserts. Temperatures west of the mountains will gradually decrease into next week, but the heat will remain through Thursday in the deserts. Slight chances of isolated showers and thunderstorms remain over the San Bernardino and Riverside mountains, locally into the High Desert Monday afternoon. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Today is off to a hot start from the coast to the deserts. At 9 AM coastal areas are running 10 to 15 degrees warmer than 9 AM yesterday. The valleys and High Desert are running 3 to 7 degrees, locally up to 10 degrees warmer than yesterday morning. The mountains and portions of the Coachella Valley are running a couple degrees cooler than yesterday morning due to the increase in moisture this morning. The moisture surge made it into Southern California overnight and that is reflected by a noticeable increase in precipitable water in the atmosphere on the 12Z KNKX sounding. This morning the precipitable water value was 1.68 inches, up almost an inch from the 0.70 inch reading on yesterday morning`s sounding. The shower and thunderstorm forecast today is shaping up to be a tricky one. Most of the high resolution model guidance is showing very isolated to no convection over our forecast area today. The ingredients of available moisture (precipitable water of 1.68 inches) and instability (most unstable CAPE was 585 J/kg on the 12Z sounding this morning) are there but the limiting factor is a large scale lifting mechanism. The mountains act as a natural lifting mechanism so the best chances of shower and thunderstorm development is over the mountains this afternoon. There is also a chance showers and potentially thunderstorms could drift east into the deserts. Model guidance is showing the moisture over Southern California very slowly decreasing into Wednesday. Guidance is also indicating the atmosphere will slowly stabilize into Wednesday. Current forecast leaves chances of isolated showers and thunderstorms in the San Bernardino and Riverside mountains into portions of the High Desert Monday afternoon. Today we`ll evaluate if those chances need to be included on Tuesday as well. A Pacific trough will slowly move into the Pacific Northwest this week, weakening the ridge currently over the Southwest. As a result temperatures west of the mountains will fall a couple degrees each day Monday through Thursday. By Thursday, even the deserts will see a few degrees of cooling. As the trough approaches and the influence from the ridge weakens, the marine layer will be able to rebuild. By mid-week night and morning low clouds should return to the coast, locally into the western valleys. && .AVIATION... 231550Z...Coast/Valleys...SCT low clouds with bases 700-1000 ft MSL coastal waters and locally onshore through 17Z. Tops will be 1200 ft MSL. There is a less than 10 percent chance of TS 18Z to 02Z Mon. Otherwise, SCT-BKN clouds mostly at/above 9000 ft MSL through this evening. A very patchy marine layer low cloud pattern is expected to continue into Monday with bases 700-1000 ft MSL coastal waters and locally onshore through 17Z. Tops will be 1200 ft MSL. Mountains/Deserts...There is a 20-30 percent chance of TSRA between 17Z and 04Z Mon, with CB tops to 35000 ft MSL and possible strong up/downdrafts and local surface wind gusts over 30 kt. Otherwise, SCT-BKN clouds mostly at/above 9000 ft MSL through this evening. Local west winds with gusts 25-35 kt will develop after 22Z, continuing this evening on the desert mountain slopes and into the deserts. && .MARINE... No hazardous marine conditions are expected through Thursday. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...Heat Advisory until 8 PM PDT this evening for Orange County Inland Areas-San Bernardino and Riverside County Valleys- The Inland Empire-San Diego County Valleys. Excessive Heat Warning until 8 PM PDT Thursday for Apple and Lucerne Valleys-Coachella Valley-San Diego County Deserts- San Gorgonio Pass Near Banning. PZ...None. && $$ PUBLIC...CO AVIATION/MARINE...Small